• 4.2 baseline — 0.6 × season 3.6 (16 gm) + 0.4 × last 10 5.1
• −0.0 opponent leak — they concede 185.4/game vs a 185.9 league average
• −0.6 at this ground — averages 3.6 there (10 gm)
• −0.5 wet forecast — averages 3.7 in the wet (13 gm)
• vs them: averages 4.6 over 7 games — evidence, never summed

Derived, not measured — plain arithmetic over his real games, shown in full. Adjustments can overlap (a game at this ground can also be a wet one); they add simply, and this note is the admission.